This invention relates to solid state polymerization of low diffusion resistance granules of polyester prepolymers.
High molecular weight polyesters, such as poly(ethylene terephthalate) (PET) and poly(ethylene napthalate) (PEN), are usually produced by a combination of melt polymerization and solid state polymerization (SSP) processes. Polyester prepolymers with relatively low molecular weight (evidenced by a relatively low intrinsic viscosity, "I.V.") are produced in a melt polymerization process. The prepolymer melt is typically extruded through a die with multiple orifices to form molten strands that are quenched, solidified, and chopped into granules or pellets. There are, of course, other methods to convert polyester melt into granular form. Hereafter, for convenience, polyester prepolymer granules, regardless of their shape or method of forming, will be referred to as pellets. The prepolymer pellets are further polymerized in solid phase to a desired high molecular weight or I.V., at temperatures below the melting point but above the glass transition temperature.
Generally, the shape of the orifices in the extrusion die used to pelletize prepolymer is round, but orifices of other simple shapes can be used. Drooping of the molten prepolymer strands and/or some flattening actions of the pelletizing equipment, cause the cross-section of the resulting prepolymer pellets to be somewhat elongated and not exactly round; i.e., approximately elliptical.
Solid state polymerization (SSP) is conducted under vacuum or in a stream of purge gas such as nitrogen. Overall, SSP involves two major steps: (1) chemical reactions and (2) diffusion of reaction by-products. The chemical reactions are transesterification and esterification.
Transesterification generates ethylene glycol (EG) as the by-product and esterification generates water as the by-product. To force the polyester prepolymer to further polymerize, reaction by-products must be effectively removed as they are generated. By-products generated by the reactions diffuse from the interior to the surfaces of the pellets where they are removed by vacuum or an inert gas stream. Since resistance to by-product diffusion from the surfaces of the pellets to the bulk of the gas phase is negligible, only the chemical reaction rates and by-product diffusion from the interior to the surfaces of the pellets are major factors in polymerization performance.
Enhanced chemical reaction rates are achieved by using the maximum allowable reaction temperature, slightly below the temperature at which polymer pellets start to stick together or to the reactor wall.
Resistance to diffusion of by-products can be reduced by reducing prepolymer particle size. However, at a fixed temperature, smaller particles have higher tendencies to stick. Therefore, lower reaction temperatures are required if particle size is reduced. Moreover, excessively small particles are hard to handle. Therefore, it is no accident that most of the polyester pellets weigh between 0.015 to 0.03 grams per pellet. Another way to reduce the by-product diffusional resistance while maintaining the pellet size is to shape the pellet cross-section like stars, crosses, dog bones and the like. However, such shapes are generally undesirable because the increased inter-pellet contact area of these shapes increases the tendency to stick. Therefore, practically all standard prepolymer pellets have round, elliptical, square, rectangular or other simple cross-sections.
Heretofore, two methods have been proposed to reduce diffusional resistance to by-products while maintaining a suitable prepolymer particle size and without increasing polymer sticking tendency. U.S. Pat. No. 3,586,647 proposes foamed pellets which are formed by incorporating nitrogen or a foaming agent into the prepolymer melt before pelletizing. Foamed pellets polymerize only slightly faster than solid pellets. In U.S. No. 4,755,587 it is proposed to use porous pellets with interconnected voids. Although porous pellets solid state polymerize much faster than standard pellets, the formation of porous pellets requires expensive additional steps, such as grinding, compacting, granulating, and classifying. Moreover, porous pellets tend to generate large amounts of fines.
It is the purpose of this invention to improve the polymerization process by providing an improved polyester prepolymer pellet form that offers reduced diffusion resistance to the reaction by-products and, therefore, higher polymerization rates, without the disadvantages of existing techniques.